The need for communication bandwidth capacity has increased dramatically in the last two decades and continues on an exponential growth path. To fill this need communications companies have invested large sums into developing infrastructures to transmit information. One of the various methods of transmitting large quantities of information that has experienced much growth in the last decade utilizes optical fibers and transmits information in the form of modulated optical signals through these fibers. A communication system using optical fiber uses transmitters at one end that typically convert electrical signals into optical signals that are transmitted through the fiber, and receivers that convert optical signals into electrical signals at the other end of the fiber carrying the optical signal.
Typically, a fiber optic transmitter uses a laser diode or other light emitting device (LED) to optically encode information and generate an optical output at various optical wavelengths, e.g., 850 nm, 1310 nm, 1550 nm etc. The optical fiber transmits the encoded information in optical form to a receiver which then converts the optical signal to an electrical signal. The optical fiber may be either single-mode or multi-mode. Typical receivers incorporate optoelectronic transducers such as photo-detectors to convert the optical signal to an electrical signal. A data demodulator then converts the data back into its original electrical form.
In order to increase transmission rates, a method of transmission known as wavelength division multiplexing (WDM) was developed for sending several different signals through a single fiber at different wavelengths. In WDM, different wavelength channels are multiplexed in the optical domain. A WDM system components include a multiplexing apparatus at the transmitting end of the WDM system to allow different wavelengths channels to be joined into a composite output signal for transmission and a de-multiplexing apparatus at the receiving end of the WDM system to allow different wavelengths channels to be separated back into their original signals. The farther apart the wavelengths channels are, the easier it is to design and fabricate the multiplexing and de-multiplexing hardware.
There are two commonly used WDM technologies, namely, coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM). Coarse wavelength division multiplexing (CWDM) is typically used up to 16 channels and dense wavelength division multiplexing (DWDM) allow up to several hundreds of signals to be combined into a single fiber. DWDM allow a multiple wavelength transmission in the C-Band (1550 nm) and more recently in the S-Band and L-Band as well. CWDM schemes have been used in many wavelength bands including near 850 nm, 1300 nm and all bands at 1500 nm. CWDM can also be used to emulate 10 Gbits/second data transmission by multiplexing 4 signals having different wavelengths, each with a data rate of 2.5 Gbits/second.
Typically, laser diodes are used at the transmitting end of a WDM system to convert a multiplexed electrical signal into an optical signal at to be transmit the optical signal into an optical fiber. Laser diodes used for WDM systems are predominantly distributed-feedback (DFB) chips. In practice, such lasers use costly packaging techniques (butterfly housings with thermoelectric coolers) to couple the light of the laser chip to the fiber and prevent the wavelength from drifting. One individual package is used for each laser, and an additional package is used for the wavelength multiplexer. The cost of packaging optical components severely affects the overall cost effectiveness of fiber optic communication systems. As much as eighty percent of the cost of WDM optical component is generally tied up in packaging. Similar problems also exist for photo detectors used at the receiving end of the WDM system.
Despite this high costs, fiber optic based solutions dominate long-haul communications because of the unsurpassed bandwidth and low loss advantages of optical fiber. However, in access applications, or in metro area applications, where there are other viable copper-based alternatives, the cost of packaging optical components severely limits the competitiveness of fiber optic systems against copper solutions. Therefore it is important to find cost effective solutions for packaging optical components.